For the purpose of reducing the weight and cost, the application of seam (electric resistance) welded steel pipes to automobile parts has been considered. Conventional seam welded steel pipes, however, have not been sufficient in workability. Bending is employed to manufacture, e.g., undercarriage or suspension parts of automobiles. When the conventional seam welded steel pipes are subjected to the bending, a problem has been experienced in that a pipe wall is greatly thinned on the outer side of a bent portion, and in the worst case a pipe is ruptured. Even in the case of not causing a rupture, a large rate of thinning of the pipe wall requires the use of a material having a greater thickness to satisfy the design stress, and therefore a sufficient reduction in weight cannot be achieved.
As disclosed in Japanese Unexamined Patent Application Publication No. 55-56624, for example, it is known that improving an r-value (Lankford value) of a pipe in the axial direction is effective to overcome the problems described above. As a method for increasing the r-value of a steel pipe, however, it is only known to increase the r-value of strip steel as a base material of a steel pipe as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 6-41689. When producing seam welded steel pipes, there has been a problem that the r-value is reduced in a portion where melting or transformation of a steel material has occurred during seam welding. Another problem has arisen in that the seam welding cannot be applied to steel plates not having a high r-value, such as hot-rolled steel plates, high tensile strength steel plates, and low, medium and high carbon steel plates.
Accordingly, it could be advantageous to provide a steel pipe being superior in workability, particularly in bending workability, in which an r-value of the pipe in the axial direction in a portion where melting or transformation of a steel material has occurred during seam welding is as high as comparable to that in a portion where melting or transformation of the steel material has not occurred, and a method of producing the steel pipe.